Conventional techniques for producing an aromatic polycarbonate oligomer from a tetrahalogenated bisphenol through interfacial polymerization include a process for producing a halogenated aromatic polycarbonate oligomer in the presence of a chain terminator as disclosed in Japanese Patent Application (OPI) No. 52834/73 (the term "OPI" as used herein means "unexamined published Japanese patent application") and a process for producing a halogenated aromatic polycarbonate oligomer without using a chain terminator as disclosed in Japanese Patent Application OPI No. 84894/76. The former process produces virtually no chloroformate compounds. According to the latter process, the resulting polycarbonate oligomer carries a hydroxyl group other than a chloroformate group and also has a broad molecular weight distribution.
On the other hand, a dichloroformate of a bisphenol can be prepared by a process described in Macromol. Chem., Vol. 57, 1 (1962), which comprises dissolving a bisphenol and excess phosgene in toluene, adding N,N-dimethylaniline, etc., to the solution to effect reaction at a low temperature around -10.degree. C. for 5 to 6 hours, and driving the excess of phosgene out of the reaction system to obtain a dichloroformate. However, this technique entails high cost arising from the use of low temperatures and the like and is not, therefore, practical.
Under these circumstances, since there has been proposed no process for producing a dichloroformate monomer of a tetrahalogenated bisphenol in a high yield at a high content under practical conditions, it has been difficult to obtain a copolycarbonate oligomer of high regularity from a tetrahalogenated bisphenol and a general bisphenol.